On Sat, Jul 30, 2016 at 05:38:27PM +0900, Namhyung Kim wrote:
> Hello,
>
> On Thu, Jul 28, 2016 at 02:08:41PM +0100, Daniel P. Berrange wrote:
> > On Thu, Jul 28, 2016 at 01:56:07PM +0100, Stefan Hajnoczi wrote:
> > > On Thu, Jul 28, 2016 at 02:39:53PM +0900, Namhyung Kim wrote:
> > > > On Thu, Jul 28, 2016 at 03:02:54AM +0300, Michael S. Tsirkin wrote:
> > > > > On Thu, Jul 28, 2016 at 12:08:30AM +0900, Namhyung Kim wrote:
> > > > > > +static ssize_t virtio_pstore_do_write(VirtIOPstore *s, struct iovec *out_sg,
> > > > > > + unsigned int out_num,
> > > > > > + struct virtio_pstore_req *req)
> > > > > > +{
> > > > > > + char path[PATH_MAX];
> > > > > > + int fd;
> > > > > > + ssize_t len;
> > > > > > + unsigned short type;
> > > > > > + int flags = O_WRONLY | O_CREAT;
> > > > > > +
> > > > > > + /* we already consume the req */
> > > > > > + iov_discard_front(&out_sg, &out_num, sizeof(*req));
> > > > > > +
> > > > > > + virtio_pstore_to_filename(s, path, sizeof(path), req);
> > > > > > +
> > > > > > + type = le16_to_cpu(req->type);
> > > > > > +
> > > > > > + if (type == VIRTIO_PSTORE_TYPE_DMESG) {
> > > > > > + flags |= O_TRUNC;
> > > > > > + } else if (type == VIRTIO_PSTORE_TYPE_CONSOLE) {
> > > > > > + flags |= O_APPEND;
> > > > > > + }
> > > > > > +
> > > > > > + fd = open(path, flags, 0644);
> > > > > > + if (fd < 0) {
> > > > > > + error_report("cannot open %s", path);
> > > > > > + return -1;
> > > > > > + }
> > > > > > + len = writev(fd, out_sg, out_num);
> > > > > > + close(fd);
> > > > > > +
> > > > > > + return len;
> > > > >
> > > > > All this is blocking VM until host io completes.
> > > >
> > > > Hmm.. I don't know about the internals of qemu. So does it make guest
> > > > stop? If so, that's what I want to do for _DMESG. :) As it's called
> > > > only on kernel oops I think it's admittable. But for _CONSOLE, it
> > > > needs to do asynchronously. Maybe I can add a thread to do the work.
> > >
> > > Please look at include/io/channel.h. QEMU is event-driven and tends to
> > > use asynchronous I/O instead of spawning threads. The include/io/ APIs
> > > allow you to do asynchronous I/O in the event loop.
> >
> > That is true, except for I/O to/from plain files - the QIOChannelFile
> > impl doesn't do anything special (yet) to make that work correctly in
> > non-blocking mode. Of course that could be fixed...
>
> Yep, I don't know how I can use the QIOChannelFile for async IO.
> AFAICS it's just a wrapper for normal readv/writev. Who is
> responsible to do these IO when I use the IO channel API? Also does
> it guarantee that all IOs are processed in order?
I'd suggest just using QIOChannelFile regardless - we need to fix the
blocking behaviour already for sake of the qemu chardev code, and your
code just adds more pressure to fix it. I/O will be done in the order
in which you make the calls, as with regular POSIX I/O funcs you're
currently using.
Regards,
Daniel
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